Lubricating oil compositions containing esters of polycarboxylic aromatic acids



LUBRICATING 01L COMPOSITIONS CONTAINING ESTERS OF POLYCARBOXYLIC AROMATIC ACIDS William J. Craven, Elizabeth, Stephen J. Metro, Scotch Plains, and Alfred H. Matuszak, Westfield, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Filed Jan. 2, 1958, Ser. No. 706,593

Claims. (Cl. 252-57) This invention relates to alcohol esters of aromatic polycarboxylic acids and to lubricating oil compositions containing said esters. Particularly, it relates to synthethic esters prepared from mono or dihydric alcohols and aromatic acids having 3 to 4 carboxylic groups per molecule, which esters are useful as synthetic lubricating oils and as oil additive materials.

The use of various aliphatic diesters and complex synthetic esters as lubricating oils is well known to the art and have been described in numerous patents, e.g. US. 2,723,286; 2,743,234; and 2,575,196. In general, these prior aliphatic synthetic ester lubricating Oils are characterized by viscosity properties that are outstanding at both low and high temperatures, especially when compared to mineral oils. Because of these characteristics, the synthetic ester oils have become of increasing importance in the field of lubrication, and one of the most important current applications of such compounds is in the lubrication of aviation gas-turbine systems such as are used in the turbo-jet or turbo-prop type of aircraft. However, in general, the load-carrying ability of the aliphatic ester oils is not particularly high. Because of the increasing severity of the conditions prevailing in the lubricaton of aviation gas-turbine systems, it is highly desirable to form synthetic ester lubricating compositions having higher load-carrying ability than is now generally available, and yet at the same time being noncorrosive. It has now been found that aromatic triand tetra-carboxylic acids, when fully esterified with aliphatic hydroxy materials, form esters which have extremely high load-carrying ability and also tend to inhibit corrosion of lead-containing bearings frequently used in aircraft engines. In addition, these aromatic esters have many of the properties which have made the aliphatic esters (e.g. di-Z-ethylhexyl sebacate) outstanding lubricants. Thus, generally speaking, the tetraesters of the invention, particularly when branched, have low pour points, good viscositytemperature relationship, high flash points, and are unusually thermally stable. Furthermore, these esters have much lower coking values than many of the aliphatic esters used as lubricants and often have much lower evaporation losses and generally speaking are useful as lubricants per se. It has been further found that when the tetraesters of the invention are used in mineral oil crankcase lubricants that they have a marked tendency to reduce engine deposits. The partial esters of the invention have many of the properties of the tetraesters, particularly With regard to inhibiting lead corrosion and imparting load-carrying ability. However, since the parital esters generally have rather high pour points, they are best used blended with other oils.

States Patent 0 3,$l9,i88 Patented Jan. 30, 1962 The esters of the invention include mono-, di-, t-ri-, and tetraesters having the general formula:

wherein R represents an aromatic nucleus having 6 carbon atoms in the ring, R is an alkyl radical containing about 3 to about 20, e.g. 5 to 13, carbon atoms, n is a number of l to 4, while In is a number of 0 to 3, such that m+n=3 if the aromatic acid is tribasic, or m+n=4 if the aromatic acid is tetrabasic.

The aromatic polycarboxylic acids used in preparing the above type of esters include the following:

0 O OH C O OH O O OH (trimellitic) (trimesie) H 0 O C C O OH 0 O OH O O OH O O OH O O OH O O OH (hemimellitic) (mellophanic) COOH HOOCQOOOH C O OH H O O C C O OH O O 0 H (pyromellitic) (prehnitie) H O O C C 0 OH O 0 OH 0 O OH Where they exist, the corresponding partial or full acid anhydrides may also be used; e.g. pyromellitic acid anhydride which has the structure:

Esters may also be prepared from the corresponding cycloheXane polycarboxylic acids that result when the aromatic acid is catalytic hydrogenated to saturate the ring. However, the esters of the invention are preferably prepared from pyromellitic acid or its anhydride since it is readily available in commercial quantities and undergoes esterification very readily.

Preferred alcohols for forming the ester are those alkanols of about 3 to about 20, e.g. 5 to 10 carbon atoms. These alcohols may be either straight chain or branched chain alcohols. Primary alcohols are particularly preferred. Secondary and tertiary alcohols, while operable, are less preferred for the purposes of this invention, since esters prepared from such alcohols generally have poorer thermal stability than the corresponding esters prepared from primary alcohols. Among the straight chain primary alcohols operable in preparing the synthetic lubricants of this invention may be mentioned n-decyl alcohol, n-octyl alcohol, n-heptyl alcohol, n-hexyl alcohol, n-amyl alcohol, n-butyl alcohol, and Fischer- Tropsch synthesis alcohols. Operable branched chain primary alcohols include 2-ethylhexyl alcohol, 2-ethylbutyl alcohol, and the Oxo alcohols. These Oxo alcohols are Well known in the art. They are prepared in a two 3 stage reaction. The first stage involves reacting olefins, such as polymers and copolymers of C and C monoolefins, with carbon monoxide and hydrogen at temperatures about 300 F. to 400 F. and pressures of about 30 to 400 atmospheres, in the presence of a suitable catalyst, ordinarily a heavy metal carbonyl, such as cobalt carbOnyl, to form a mixture of aldehydes having one carbon atom more than the olefin. In the second stage, the aldehyde mixture is hydrogenated, to form an isomeric mixture of highly branched chain primary alcohols which is recovered by distillation. The process is Well known and has been described in various US. Patents, e.g. US. 2,327,066 and US. 2,593,428.

The aromatic acid esters of the invention are prepared by conventional esterification techniques. The esterification is carried out by reacting l to 4 molar proportions of an alcohol (depending upon whether a full or partial ester is desired) per one molarproportion of the aromatic acid, under reflux conditions. Generally, a water-entraining agent, e.g. heptane, toluene, etc., is used, and the reaction is carried on until the calculated amount of water is removed overhead. When the full ester is desired, a slight excess of alcohol can be used in order to insure completion of the reaction. Esterification catalysts may be used, e.g. sodium bisulfate, sulfuric acid, toluene sulfonic acid, sodium methylate, calcium oxide, etc., although the reaction may be carried out without a catalyst. In any case, after the desired amount of water is removed, the remaining reaction product may be filtered and washed, if a catalyst was used, and distilled under vacuum in order to remove the entraining agent and any un'reacted alcohol. Frequently, it is desirable to carry out the esterification without a catalyst. This eliminates the necessity for filtering and washing to remove the catalyst which otherwise might tend to make the product corrosive.

The resulting ester product may be used as a lubricant per se or it can be advantageously blended in any proportions with other lubricating oils. The full esters of the higher molecular weight alcohols, particularly the Oxo alcohols, seem to have the best all around properties for use as a lubricant per se, while the partial esters are best used by blending with other oils. Thus, such blends may contain for example, about 0.25 to 90, preferably 0.25 to 70, weight percent of the aromatic acid ester and about 99.5 to 10, preferably 99.5 to 30 weight percent of another lubricating oil. The lubricating oil used with the aromatic acid esters may be a mineral lubricating oil, a syn thetic lubricating oil, or any mixtures thereof. Particularly preferred synthetic oils for blending with the aromatic acid ester materials are the saturated aliphatic diesters represented by the formula:

wherein R is a straight or branched chain hydrocarbon radical of a C to C alkandioic acid, while R represents an alkyl radical of a C to C branched or straight chain alkanol, and the total number of carbon atoms in the molecule being twenty or more. Specific examples of such diesters include di(2-ethylhexyl)sebacate, C Oxo alcohol ester of C Oxo acid, di(C Oxo)adipate, etc. Other synthetic oils which may be used will include esters of monobasic acids (e.g. C OX alcohol ester of C Oxo acid), esters of glycols (e.g. C Oxo acid diester of tetraethylene glycol), complex esters, esters of phosphoric acid, halocarbon oils, sulfite esters, silicone oils, carbonates, formals, polyglycol-type synthetic oils, etc.

Various other additives may also be added to the lubricating compositions of the invention in amounts of about 0.1 to 10.0 weight percent, based on the total weight of the composition. For example, detergents such as calcium petroleum sulfonate; oxidation inhibitors such as phenylalpha-napthylamine or phenolthiazine; corrosion inhibitors such as sorbitan monooleate; pour point depressants; dyes; grease thickeners; load-carrying agents and the like may be added.

The invention will be further understood by the following examples:

EXAMPLE I A. A tetra C Oxo ester of pyromellitic acid was prepared as follows:

Into a 1000 ml. round bottom three-necked flask fitted with a stirrer, thermometer, and a reflux condenser with a water trap, was placed 215 grams (1.65 moles) of C Oxo alcohol (prepared by subjecting butylene-propylene copolymers to the 0x0 process) and 104 grams (0.41 mole) of finely powdered pyromellitic acid was added. Three grams of sodium hydrogen sulfate as a catalyst and 15 ml. of heptane as a water entraining agent were next introduced. The mixture was then refluxed at atmospheric pressure and stirred vigorously for three hours. During this time the pyromellitic acid gradually dissolved and reacted, while the calculated amount of water (1.64 moles) collected in the trap. The resulting brown oil residue was stripped of volatiles at 200 C. under a jet of nitrogen. The residue was filtered free of catalyst and a nearly quantitative yield was realized. The product had the following viscosity characteristics:

Vis./210 F., SUS, 84.8 Vis./l00 F., SUS, 973.3 Viscosity index, 89.5

B. 4.2 weight percent of the tetra C Oxo ester prepared above was blended with 95.8 weight percent of a commercial premium heavy duty 10W-30 motor oil. This motor oil was formulated from a low viscosity mineral oil to which was added a detergent inhibitor, viscosity index improver and a pour depressant. This composition was tested in a single cylinder, two-cycle Homelite engine for deposit forming tendencies. The engine was operated at full power volts, 13 amps, 3600 rpm.) for twenty hours; the fuel and oil are mixed together (roughly pint of oil being used per gallon of fuel) and the combination used directly to operate the engine. The test serves primarily to predict the tendency of an oil to form deposits on passenger car intake valve undersides. The piston underside demerit in the Homelite test is a measure of this; demerits of 2.0 or greater indicating the oil would give copious intake valve deposits while demerits of 1.0 or less are obtained with oils that would give satisfactory car performance in this respect. The test also gives a general indication of the detergency of the oil (i.e. its ability to prevent ring sticking and heavy varnish formation).

Results of the above tests are summarized in Table I which follows:

Table I EFFECTIVENESS OF TETRA Cs OX0 PYROMELLITIC ESTER IN REDUCING ENGINE DEPOSITS Amount of Exester added Piston No. 1 haust to oil, underside ring side Weight perdemerit sticking varcent nish None 2. 3 4 6 4. 2 1. 0 0 2 As seen from the above data, the pyromellitic ester was every effective in reducing engine deposits of mineral oil base crankcase lubricants.

EXAMPLE II OX) adipate and a di-(C Oxo) adipate containing 1 weight percent of phenothiazine as an oxidation inhibitor. Since synthetic oils are frequently used to lubricate lead-containing bearings, the resulting blends were tested for lead corrosion. This test was carried out by rapidly rotating a bimetallic strip consisting of a lead strip and a copper strip bound together, in an oil sample maintained at 325 F. while air is bubbled through the sample. The weight loss of the lead strip is then determined, and reported in terms of mg. wt. loss per square inch of lead surface. Lubricants possessing very low lead corrosion are desired for aircraft engine lubrication. The blends were also tested for load-carrying ability in the Ryder Gear Test in accordance with MIL-7808C specification procedures.

The results of the above tests are shown on the following table:

Table II Weight Lead corrosion (weight loss in mg./in.'*) percent Ryder PMDA gear load, half ester lbs/in. (0a 0x0) 1 hour 4 hours 8 hours 12 hours a in adipate base oil 1 Adipate base oil consisted of 1 weight percent of phenothiazine and 99 weight percent of a 50/50 (by volume) blend of 08 0x0 adipate and C 0x0 adipate. The Oxo portion of the adipates were derived from Oxo alcohols prepared from a C -C olefin feed.

The above table demonstrates the remarkable effectiveness of very minor amounts of the half ester of pyromellitic acid in imparting load-carrying ability and in inhibiting lead corrosion.

The 0X0 adipate-PMDA half ester-phenothiazine blends were further tested for corrosiveness to copper, magnesium, iron, aluminum and silver and for changes in viscosity and acidity. These tests were carried out in accordance with MILL-7808C specification procedure, i.e., by immersing weighed strips of the metal to be tested in 100 cc. of the sample maintained at 347 F. for 72 hours while bubbling 0.5 liter per hour of air through the sample. The metal strip is then reweighed to determine the weight change as mg./cm. and the change in viscosity and neutralization number of the composition is determined. The results of these tests are summarized in Table III which follows:

A series of tetra esters of pyromellitic anhydride were prepared in the general manner of Example I. These esters were prepared from amyl alcohol, C Oxo alcohol (prepared from copolymer of propylene and butylene), C OX0 alcohol (prepared from tripropylene), and C Oxo alcohol (prepared from tetrapropylene). The amyl alcohol was prepared using para-toluene sulfonic acid as the catalyst, while the 0x0 alcohol esters were prepared using sodium hydrogen sulfate as the catalyst. After completion of the reaction, the catalysts were removed by filtration. In the case of the 0x0 alcohol Table IV TE'IRA ESTERS OF PYROMELLITIG DI-ANHYDRIDE 0x0 alcohol esters Vise/210 F., cs Vise/ F., cs Flash point, F Fire point, F Actual pour point, F Lead corr. test:

1 hr. mg./in. 4 hrs. mg., in. 8 hrs. rug/in! 12 hrs. mg./in. Total acid nent. mg. KOH/ml. Ryder gear load, lbs./in., (avg) The tetraesters of pyromellitic dianhydride of Table IV has good viscosity-temperature properties, high flash points, high fire points and other properties which make them suitable for use as synthetic lubricants for aircraft use. To illustrate this use, a composition consisting of 100 parts by weight of tetra-C OX0 pyromellitate and 1 part by weight of phenothiazine as oxidation inhibitor was tested in a Bearing Ring Test under conditions similar to those encountered in the lubrication of the bearings supporting the rotor shaft of a Pratt and Whitney J-57 turbo-jet engine. In this test, -a 100 mm. dia. aircraft steel roller bearing rotating at 10,000 rpm. is maintained at a temperature of 350 F., while being sprayed with a jet of the oil composition heated to 250 F., at the rate of 500 cc. of oil per minute. The oil falls off the bearing into a reservoir, where it is picked up by a pump and recirculated, the total amount of oil in the system being two gallons. After 50 hours operation in the above test, the composition of the tetra-C Oxo pyromellitate and phenothiazine still appeared very clean, showing no degradation. The bearing was also in excellent condition showing no deposits.

EXAMPLE IV 25 wt. percent of amyl tetra pyromellitate was mixed with 75 wt. percent of a paraflinic mineral lubricating oil having a viscosity of 10 cs. at 210 R, and a viscosity index of 106. The blend was then tested for load-carrying ability in the Ryder gear test. The results of these tests are summarized in the following table along with similar data on the pyromellitate alone and the mineral oil alone:

Table V Ryder gear test load (lbs/in.) 75 wt. percent mineral lubricating oil-[-25 wt. percent amyl pyromellitate 3650 Arnyl pyromellitate 2900 Mineral lubricating oil 1850 It will be noted that the load-carrying ability of the blend exceeds that of either component alone. This was an unexpected improvement.

EXAMPLE V A quarter, or monoester was prepared by reacting 1 molar amount C Oxo alcohol with 1 molar amount of pyromellitic dianhydride. This ester was prepared in the general manner of Example I, but no catalyst was Load-carrying ability:

used. No neutralization in the manner described in the preparation of the esters of Example 111 was thus required. 0.25 wt. percent of this monoester was added to 99.75 wt. percent synthetic diester base lubricating composition. This diester base lubricant, in turn, consisted of 97.0 parts by Weight of di(C Oxo)azelate; 3 parts by Weight of a complex ester prepared by simultaneously reacting 1 mole of polyethylene glycol of 200 mol. wt., 2 moles of C OX alcohol and 2 moles of adipic acid; 4 parts by weight of a dimethyl silicone oil having 10 cs. viscosity at 77 F, 1 part by weight of tricresyl phosphate; 1 part by weight of phenothiazine and .003 part by weight of an anti-foamant. This composition was then tested for lead corrosiveness in the test previously described. For comparison purposes, the diester base lubricating composition containing none of the monoester of pyromellitic acid, was also tested for lead corrosion. The results of these tests are summarized in Table VI.

Table VI INHIBITION OF LEAD CORROSION BY MONO Cs OX0 PYROMELLITATE Load corrosion (weight loss in night?) lhour 4hours 8h0u1's 12hours Diester base lubricant 1.46 7.7 17.5 137 Diester base lubricant plus 0.25 weight percent mono Cs Oxo pyromellitate O O O 0 In summary, the invention relates to lubricating oil compositions comprising alkyl esters of polycarboxylic aromatic acids having 3 to 4 carboxy groups per molecule and wherein the alkyl radicals each contain 3 to 20, preferably to 13 carbon atoms. Esters of pyromellitic acid are preferred and may be prepared either from the acid or its anhydride. The normally liquid tetraesters of pyromellitic acid, particularly those prepared from C to C Oxo alcohols, are suitable as lubricating oils per se, or can be blended in any proportion with other lubricating oils. The partial esters of pyromellitic acid are frequently solids or near solids at room temperature, as are certain of the straight chain higher alkyl tetraesters, tag. the straight chain tetra octyl pyromellitate which melts at about 90 F. Such materials are best used in minor ,amounts as additives for lubricating oils to impart El.

properties thereto, to inhibit lead corrosion, to reduce engine deposits, etc. Generally, about 0.25 to 10.0% or" the partial or tetraester in lubricating oil Will suffice for these purposes, although much larger amounts may be used as previously disclosed.

What is claimed is:

1. A lubricating oil composition suitable for lubrication of aircraft engines comprising a major amount of a synthetic aliphatic ester lubricating oil and about 0.25 to 10% by weight of a partial ester selected from the group consisting or" monoalkyl pyromellitate and dialkyl pyromellitate, wherein said alkyl groups are alkyl groups of C to C alcohols.

2. A lubricating oil composition according to claim 1, wherein said partial ester is dioctyl pyromellitate and said octyl groups are branched chain.

3. A lubricating oil composition according to claim 1, wherein said partial ester is monooctyl pyromellitate and said octyl groups are branched chain.

4. A lubricating oil composition comprising a major amount of a saturated diester of a C to C alkandioic acid and a C to C alkanol, said diester being normally corrosive to lead, and a lead corrosion inhibiting amount, within the range of about 0.25 to 10.0 wt. percent, of dialkyl pyromellitate, wherein said alkyl group contains 5 to 10 carbon atoms.

5. A lubricating oil composition comprising a major amount of saturated diester of a C to C alkandioic acid and a C to C alkanol, said diester being normally corrosive to lead, and a lead corrosion inhibiting amount, within the range of about 0.25 to 10.0 wt. percent, of monoalkyl pyromellitate, wherein said alkyl group contains 5 to 10 carbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS 1,993,736 Graves et al. Mar. 12, 1935 1,993,737 Graves et al Mar. 12, 1935 1,993,738 Graves et al. Mar. 12, 1935 2,134,736 Reuter Nov. 1, 1938 2,199,187 Rosen Apr. 30, 1940 2,507,509 Fegley et a1. May 16, 1950 2,516,640 Montgomery et a1 July 25, 1950 2,545,169 Salathiel Mar. 13, 1951 2,568,965 Montgomery et al Sept. 25, 1951 OTHER REFERENCES Lubrication Engineering, August 1952, pp. 177179. 

1. A LUBRICATING OIL COMPOSITION SUITABLE FOR LUBRICATION OF AIRCRAFT ENGINES COMPRISING A MAJOR AMOUNT OF A SYNTHETIC ALIPHATIC ESTER LUBRICATING OIL AND ABOUT 0.25 TO 10% BY WEIGHT OF A PARTIAL ESTER SELECTED FROM THE GROUP CONSISTING OF MONOALKYL PYROMELLITATE AND DIALKYL PYROMELLITATE, WHEREIN SAID ALKYL GROUPS ARE ALKYL GROUPS OF C3 TO C20 ALCOHOLS. 